Bismuth-antimony (Bi100−xSbx) alloys have the highest thermoelectric figure of merit of all n-type thermoelectric materials below 200 K. They are the only Te-free thermoelectric alternatives to the tetradymite materials for applications at and below room temperature. Single-crystal Bi100−xSbx alloys show the maximum figure of merit zT∼0.5 at 200 K along the trigonal axis crystallographic direction, but the cost associated with single-crystal growth and the tendency of single crystals to cleave preclude their use. Mechanically robust polycrystalline Bi100−xSbx/Al2O3 nanocomposites are shown here to be able to reach competitive zT values. Two compositions are investigated, Bi82Sb18 and Bi88Sb12. Thermal and electrical transport properties confirm significant reduction of lattice thermal conductivity in the nanocomposite samples, but the concurrent loss of electrical conductivity leads to an unfavorable net effect on zT. In contrast, a large increase in thermopower is observed in the Bi82Sb18/Al2O3 nanocomposite system, which is attributed to a better optimized doping level. Accordingly, the zT of a Bi82Sb18/Al2O3 nanocomposite sample is shown to reach zT∼0.4 at 240 K, which rivals that of single crystals. Near room temperature, the zT of the nanocomposite sample is improved by ∼60% over that of the single-crystalline sample. Galvano- and thermomagnetic analysis suggests a strong effect of carrier concentration on the zT of Bi100−xSbx/Al2O3 nanocomposite samples.Bismuth-antimony (Bi100-xSbx) alloys have the highest thermoelectric figure of merit of all n-type thermoelectric materials below 200 K. They are the only Te-free thermoelectric alternatives to the tetradymite materials for applications at and below room temperature. Single-crystal Bi100-xSbx alloys show the maximum figure of merit zT similar to 0.5 at 200 K along the trigonal axis crystallographic direction, but the cost associated with single-crystal growth and the tendency of single crystals to cleave preclude their use. Mechanically robust polycrystalline Bi100-xSbx/Al2O3 nanocomposites are shown here to be able to reach competitive zT values. Two compositions are investigated, Bi82Sb18 and Bi82Sb18. Thermal and electrical transport properties confirm significant reduction of lattice thermal conductivity in the nanocomposite samples, but the concurrent loss of electrical conductivity leads to an unfavorable net effect on zT T. In contrast, a large increase in thermopower is observed in the Bi82Sb18/Al2O3 nanocomposite system, which is attributed to a better optimized doping level. Accordingly, the zT of a Bi82Sb18/Al2O3 nanocomposite sample is shown to reach zT similar to 0.4 at 240 K, which rivals that of single crystals. Near room temperature, the zT of the nanocomposite sample is improved by similar to 60% over that of the single-crystalline sample. Galvano- and thermomagnetic analysis suggests a strong effect of carrier concentration on the zT of Bi100-xSbx/Al2O3 nanocomposite samples.